In order to set the background image for my WM (spectrwm <3) I use hsetroot and point it to ~/.taustakuva [taustakuva means wallpaper in Finnish <3], a symbolic link in my home directory:

$ ln -sf path_to_your_picture ~/.taustakuva

Then I simply add this line to ~/.xinitrc: hsetroot -center ~/.taustakuva

For eyecandy I use compton, a lightweight and advanced X compositor forked out of xcompmgr, which gives a nice transparency effect (among others) to urxvt:
1. Add to ~/.xinitrc: compton -o1 -b
2. If using urxvt (rxvt-unicode-256colors) don’t forget to set either:

URxvt*background : [95]#your_color_code_in_hexadecimal

URxvt.transparent : true

in ~/.Xdefaults according to your taste.
You will end with something like this:

Just a fanart wallp I made some time ago that I’m using again as my GRUB background:

Real size: 2048×1152

To set a picture as GRUB’s background open /etc/default/grub and follow these steps:

Uncomment and point GRUB_BACKGROUND= to where your picture lies, i.e. /boot/grub/Arch_Rocks.png. Bear in mind that the directory where your picture reside must be mounted at the time GRUB is loaded so best place usually is /boot/{whatever};

Since I switched to spectrwm I found that I had to explicitly tell X to use the keyboard layout I find most useful.

Let’s say you’re using the US Intl. keyboard layout (the only sane layout if you ask me) and you need to enter special symbols and characters, the altgr-intl layout – aka French/English layout on RH and its siblings distros – is the way to go then as “all of the non-US keys are all hidden behind a single key: the right Alt key.”.

So in order to instruct X to use this useful keyboard layout you could:
1. Add the corresponding section to Xorg’s keyboard settings by creating the appropriate file in /etc/X11/xorg.conf.d – not really recommended as it is somewhat awkward nowadays and the least you fiddle with your admin account the less you probably end up messing it;
2. Just add setxkbmap -rules evdev -model evdev -layout us -variant altgr-int to your ~/.xinitrc file (or any other init script parsed by your WM);